Page 95 - Advances in bioenergy (2016)

of the much higher autotrophic microalgal yields of the latter, although there are substantial
differences between photobioreactors dependent on design and materials use (e.g., Refs 46,
64, 91, and references therein).

With currently operational technologies, the EROIs of lipid-based biofuels derived from
autotrophic microalgae may well be < 1 e.g. 22,61,91,95 . None of the available LCAs performed
so far for future liquid microalgal biofuels finds an EROI of more than 5, whatever the system
boundaries and type of allocation chosen (see also Table 3.2). Even with optimistic
assumptions about future biomass and lipid yields, estimated EROIs do not always turn out to
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be more than 1. A recent case in point is the paper of Brentner et al., who studied cultivation
in both open ponds and bioreactors. They suggest an EROI of less than 1 while using an
−1 93
−1
99
optimistic assumption about future lipid yield (53,200 l oil ha year ). Khoo et al., who
consider a bioreactor/pond combination, assume a yearly microalgal biomass yield of
−1
approximately 91 mg ha , a lipid content of 45% and a reduction of energy input in processing
algal culture to biodiesel by 60%, but still estimate an EROI of less than 1. Moreover, it may
be noted that currently available LCAs may often overestimate EROIs due to noninclusion
within system boundaries of energetically important aspects of the life cycle, such as water
management preceding algal cultivation (see also Table 3.2). For instance, Murphy and Allen 29
studying fresh water-fed ponds for the mass cultivation of algae in the continental United States
estimated that energy inputs associated with water management would exceed the energy
−1
−1
content of biomass produced (43.2 mg dry weight ha year ) by a factor 7. 29